diff --git a/src/tokenizer/ascii_folding_filter.rs b/src/tokenizer/ascii_folding_filter.rs
index 89ba04bc9..3e800971b 100644
--- a/src/tokenizer/ascii_folding_filter.rs
+++ b/src/tokenizer/ascii_folding_filter.rs
@@ -1527,7 +1527,7 @@ fn fold_non_ascii_char(c: char) -> Option<&'static str> {
 }
 
 // https://github.com/apache/lucene-solr/blob/master/lucene/analysis/common/src/java/org/apache/lucene/analysis/miscellaneous/ASCIIFoldingFilter.java#L187
-fn to_ascii(text: &mut str, output: &mut String) {
+fn to_ascii(text: &str, output: &mut String) {
     output.clear();
 
     for c in text.chars() {
diff --git a/src/tokenizer/lower_caser.rs b/src/tokenizer/lower_caser.rs
index af2389301..2136b0063 100644
--- a/src/tokenizer/lower_caser.rs
+++ b/src/tokenizer/lower_caser.rs
@@ -22,7 +22,7 @@ pub struct LowerCaserTokenStream<'a> {
 }
 
 // writes a lowercased version of text into output.
-fn to_lowercase_unicode(text: &mut str, output: &mut String) {
+fn to_lowercase_unicode(text: &str, output: &mut String) {
     output.clear();
     for c in text.chars() {
         // Contrary to the std, we do not take care of sigma special case.